Flame scanner having non-linear amplifier with temperature compensation

ABSTRACT

An amplifier assembly ( 100 ) includes an amplifier ( 102 ) having an input terminal, an output terminal and a feedback terminal; a first feedback path connecting the output terminal to the feedback terminal; a second feedback path connecting the output terminal to the feedback terminal; a switch ( 124 ) positioned in the second feedback path, the switch ( 124 ) opening or closing in response to a voltage at the output terminal relative to a breakpoint, when the switch ( 124 ) is open, the amplifier assembly ( 100 ) has a first gain and when the switch ( 124 ) is closed, the amplifier assembly ( 100 ) has a second gain; and a thermally variable element ( 152 ) connected to the switch ( 124 ), the thermally variable element ( 152 ) configured to generate a compensation voltage to maintain the breakpoint in response to varying temperature of the switch ( 152 ).

TEHCNICAL FIELD

The subject matter disclosed herein relates generally to the field offlame scanners, and more particularly, to a flame scanner having anon-linear amplifier with temperature compensation.

BACKGROUND

Flame scanners are used to detect the presence of a flame in equipmentsuch as furnaces, boilers, etc. An amplifier is typically used toamplify the output signal from a flame sensor. In many cases, the inputsignal into an amplifier varies greatly. The amplifier needs a high gainin order to process an input signal at a lower range (e.g., lowvoltage). On the other hand, if the input voltage is high, the amplifierneeds a low gain in order to prevent the amplifier from going intosaturation.

BRIEF DESCRIPTION

According to one embodiment, an amplifier assembly includes an amplifierhaving an input terminal, an output terminal and a feedback terminal; afirst feedback path connecting the output terminal to the feedbackterminal; a second feedback path connecting the output terminal to thefeedback terminal; a switch positioned in the second feedback path, theswitch opening or closing in response to a voltage at the outputterminal relative to a breakpoint, when the switch is open, theamplifier assembly has a first gain and when the switch is closed, theamplifier assembly has a second gain; and a thermally variable elementconnected to the switch, the thermally variable element configured togenerate a compensation voltage to maintain the breakpoint in responseto varying temperature of the switch.

In addition to one or more of the features described above, or as analternative, further embodiments may include wherein the first feedbackpath includes a first resistance.

In addition to one or more of the features described above, or as analternative, further embodiments may include wherein the second feedbackpath includes a second resistance.

In addition to one or more of the features described above, or as analternative, further embodiments may include wherein when the switch isopen, the first gain is in response to the first resistance.

In addition to one or more of the features described above, or as analternative, further embodiments may include wherein when the switch isclosed, the second gain is in response to the first resistance and thesecond resistance.

In addition to one or more of the features described above, or as analternative, further embodiments may include wherein the first feedbackpath and second feedback path are in electrical parallel.

In addition to one or more of the features described above, or as analternative, further embodiments may include wherein the switch is atransistor.

In addition to one or more of the features described above, or as analternative, further embodiments may include wherein the transistor is aMOSFET.

In addition to one or more of the features described above, or as analternative, further embodiments may include wherein the thermallyvariable element is a diode.

In addition to one or more of the features described above, or as analternative, further embodiments may include wherein thermally variableelement contributes a compensation voltage, the compensation voltagedecreasing with increasing temperature.

According to another embodiment, a flame scanner includes an amplifierhaving an input terminal, an output terminal and a feedback terminal; afirst feedback path connecting the output terminal to the feedbackterminal; a second feedback path connecting the output terminal to thefeedback terminal; a switch positioned in the second feedback path, theswitch opening or closing in response to a voltage at the outputterminal relative to a breakpoint, when the switch is open, theamplifier assembly has a first gain and when the switch is closed, theamplifier assembly has a second gain; and a thermally variable elementconnected to the switch, the thermally variable element configured togenerate a compensation voltage to maintain the breakpoint in responseto varying temperature of the switch.

Technical effects of embodiments of the disclosure include a flamescanner that includes non-linear amplifier having temperaturecompensation.

These and other advantages and features will become more apparent fromthe following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a flame sensing system in an embodiment;

FIG. 2 is a schematic diagram of a non-linear amplifier assembly in anembodiment; and

FIG. 3 is a plot of gain of the non-linear amplifier assembly in anembodiment.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a flame sensing system 10 in an embodiment.The flame sensing system 10 includes a flame scanner 12 including asensor 14 and a signal conditioner 20. The sensor 14 may be a photodiodeor other known type of flame sensor with a voltage output. The sensor 14generates a detection signal in the presence of a flame. The sensor 14may selectively generate a detection signal in response to certainwavelengths of light, such that the sensor 14 only generates a detectionsignal when a flame is present. The signal conditioner 20 receives thedetection signal from the sensor 14 and generates an output signal thatis used by a controller 50 to determine the presence of a flame. Thesignal conditioner 20 includes a non-linear amplifier assembly toamplify the detection signal generated by sensor 14. As the detectionsignal from sensor 14 can vary from a low level to a high level, theamplifier assembly employs a non-linear gain.

FIG. 2 is a schematic diagram of an amplifier assembly 100 in anembodiment. The amplifier assembly 100 includes an input, Vin, thatreceives the detection signal from sensor 14. An amplifier (e.g., anoperational amplifier) receives the detection signal from sensor 14 at anon-inverting input terminal. The output at an output terminal ofamplifier 102, Vout, may be provided to controller 50, or may be furtherprocessed by signal conditioner 20.

The amplifier assembly 100 uses two feedback paths from the outputterminal of the amplifier 102 to a feedback terminal, e.g., theinverting input of the amplifier 102. A first feedback path 110 includesa first resistance 112. The first resistance 112 establishes a firstgain for the amplifier assembly 100. A second feedback path 120 includesa second resistance 122. The second feedback path 120 is in parallelwith the first feedback path 110. The second feedback path 120 alsoincludes a switch 124, in the form of a MOSFET transistor having a gateterminal connected to the output of amplifier 102, with the drain andsource terminals in series with the second feedback path 120.

The non-linear gain of amplifier assembly 100 is depicted in FIG. 3. Thefirst gain is shown as linear segment A1 and the second gain is shown aslinear segment A2. The first gain is greater than the second gain, asindicated by the slope of A1 being greater than the slope of A2. Abreakpoint, BP, identifies the transition between the first gain and thesecond gain.

In operation, when the output of the amplifier 102, Vout, is below thebreakpoint, BP, switch 124 is open, and the gain of the amplifierassembly 100 is dictated by the first feedback path 110. In this mode,the gain of the amplifier assembly 100 is dictated by the firstresistance 112. When the output of amplifier 102, Vout, is above thebreakpoint, BP, switch 124 is closed, and the gain of the amplifierassembly 100 is dictated by the first feedback path 110 in parallel withthe second feedback path 120. In this mode, the gain of the amplifierassembly 100 is dictated by the first resistance 112 and the secondresistance 122 in electrical parallel. The second gain is lower than thefirst gain, as the net resistance of the first resistance 112 and thesecond resistance 122 in parallel is lower than the first resistance112.

The amplifier assembly 100 also includes a temperature compensationelement 152, which maintains the breakpoint, BP, at a consistent voltageeven when the temperature of the amplifier assembly 100 varies. Thevoltage at the gate terminal of switch 124 is set by a voltage divideracross resistors 156 and 154. As temperature increases, the turn-onvoltage of the switch 124 will decrease. Unless compensated for, thiswill cause the breakpoint to go down with increasing temperature, andintroduce unwanted variance in the gain of the amplifier assembly 100.To compensate for temperature variations, a temperature compensationelement 152 is connected to the gate terminal of the switch 124, inseries with and between the resistors 156 and 154. In FIG. 2, thethermally variable element 152 is a diode. The voltage at the gateterminal of switch 124 is Vout(R2/(R1+R2))+Vcomp, where Vcomp is acompensation voltage provided by the thermally variable element 152. Itis understood that the thermally variable element 152 may be implementedwith other devices, and embodiments are not limited to use of a diode.

In the example of FIG. 2, the compensation voltage is the forward biasvoltage drop across the diode 152. In operation, as temperatureincreases, the forward bias voltage of diode 152 decreases. This lowersthe voltage at the gate terminal of switch 124, so that the Voutbreakpoint, BP, remains substantially constant even with varyingtemperature.

The amplifier assembly 100 provides two distinct gains, A1 and A2. Whenthe detection signal is low (e.g., Vout less than the breakpoint), theamplifier assembly 100 provides a first gain. When the detection signalis high (e.g., Vout greater than the breakpoint), the amplifier assembly100 provides a second gain. The second gain is lower than the firstgain.

While the disclosure has been described in detail in connection withonly a limited number of embodiments, it should be readily understoodthat the disclosure is not limited to such disclosed embodiments.Rather, the disclosure can be modified to incorporate any number ofvariations, alterations, substitutions or equivalent arrangements notheretofore described, but which are commensurate with the spirit andscope of the disclosure. Additionally, while various embodiments of thedisclosure have been described, it is to be understood that aspects ofthe disclosure may include only some of the described embodiments.Accordingly, the disclosure is not to be seen as limited by theforegoing description, but is only limited by the scope of the appendedclaims.

1. An amplifier assembly comprising: an amplifier having an inputterminal, an output terminal and a feedback terminal; a first feedbackpath connecting the output terminal to the feedback terminal; a secondfeedback path connecting the output terminal to the feedback terminal; aswitch positioned in the second feedback path, the switch opening orclosing in response to a voltage at the output terminal relative to abreakpoint, when the switch is open, the amplifier assembly has a firstgain and when the switch is closed, the amplifier assembly has a secondgain; and a thermally variable element connected to the switch, thethermally variable element configured to generate a compensation voltageto maintain the breakpoint in response to varying temperature of theswitch.
 2. The amplifier assembly of claim 1 wherein the first feedbackpath includes a first resistance.
 3. The amplifier assembly of claim 2wherein the second feedback path includes a second resistance.
 4. Theamplifier assembly of claim 3 wherein when the switch is open, the firstgain is in response to the first resistance.
 5. The amplifier assemblyof claim 3 wherein when the switch is closed, the second gain is inresponse to the first resistance and the second resistance.
 6. Theamplifier assembly of claim 1 wherein the first feedback path and secondfeedback path are in electrical parallel.
 7. The amplifier assembly ofclaim 1 wherein the switch is a transistor.
 8. The amplifier assembly ofclaim 7 wherein the transistor is a MOSFET.
 9. The amplifier assembly ofclaim 1 wherein the thermally variable element is a diode.
 10. Theamplifier assembly of claim 1 wherein thermally variable elementcontributes a compensation voltage, the compensation voltage decreasingwith increasing temperature.
 11. A flame scanner including the amplifierassembly of claim 1.